The Mexican–American War, also known in the United States as The Mexican War and in Mexico as la intervención norteamericana (the North American Intervention), was a military conflict fought between the United States and Mexico from 1846 to 1848, in the wake of the 1845 U.S. annexation of Texas. Mexico had not recognized the secession of Texas in 1836 and announced its intention to take back what it considered a rebel province.

In the United States, the war was a partisan issue, supported by most Democrats and opposed by most Whigs, with popular belief in the Manifest Destiny of the United States ultimately translating into public support for the war. In Mexico, the war was considered a matter of national pride.

The widespread view that America's "destiny" was to become a continental nation stretching to the Pacific was fed by public confidence in the nation's founding ideals of free and representative government, the development of steam power and the telegraph (1844), and additions to American territory, most notably the Louisiana Purchase. But nationalistic and, arguably, racist attitudes also justified ambitions for land. Mexico, in contrast, was recently independent from Spain and had seen a succession of weak and ineffective governments. It's northern territories were sparsely populated and its economy and industrial base were relatively undeveloped.

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The most important consequence of the war was the Mexican Cession, in which all the Mexican territories from California to southwestern Wyoming, west of Texas along the Rio Grande River and south of the 42nd parallel were ceded to the United States—almost 15 percent of the nation's total area. The United States paid $15,000,000 for the land, half of what it had offered prior to the war. Throughout subsequent history, the American Southwest has retained much of its Hispanic heritage, while growing economic inequities between the neighboring countries have encouraged widespread legal and illegal immigration of Mexicans into the United States.

Background

Prior to the Mexican-American War, what is now Texas was the northernmost province of Mexico. Texas and other northern territories of Mexico were visited by mountain men from the U.S. and tradesmen who traversed the Santa Fe Trail. U.S. citizens were already in California, coming by way of the California Trail, and U.S. ships had been exchanging goods for hides and tallow along the coast of California. For the 25 years subsequent to Mexico's independence from the Spanish Empire, this area had been a part of the first Mexican republic (1823-1861) or the First Mexican Empire (1822-1823) that preceded it. The Spanish Empire had gained these territories by conquering the Aztec Empire and various other Native American peoples.

In the years following the Louisiana Purchase by the United States, U.S. settlers began to move westward into Spanish territory, encouraged by Spanish land grants and the United States government. After the Mexican War of Independence, Mexico inherited ownership of the provinces of Alta California, La Mesilla, Nuevo Mexico and Tejas, from Spain, and the westward migration of U.S. settlers continued. Since the times of New Spain, the Spanish Crown gave permission to U.S. settlers to obtain land in Texas provided they declared themselves to be Catholic and manifested their obedience to the king.

In the mid-1830s, the government of Mexico, under General Santa Anna, attempted to centralize power. However, several Mexican states rebelled against his government, including Texas, California, San Luis Potosí, Querétaro, Durango, Guanajuato, Michoacán, Yucatán, Jalisco, and Zacatecas. Texans had multiple grievances, including the abolition of slavery by Mexico in 1829 and the abolition of the federalist Constitution of 1824 for a centralist government under Santa Anna. The violent insurgency that started in Texas is known as the Texas Revolution.

The new Mexican government, weakened and virtually bankrupt from the Mexican War of Independence, found it difficult to govern its northern territories, which in any case were hundreds of miles from the capital of Mexico City.

Republic of Texas

The Republic of Texas. The present-day outlines of the U.S. states are superimposed on the boundaries of 1836–1845.

In the successful 1836 Texas Revolution, Texas won its independence after defeating Santa Anna and the Mexican army. General Santa Anna was taken captive by the Texas militia and only released after he promised to recognize the sovereignty of the Republic of Texas. When Santa Anna returned to Mexico however, the government refused to recognize the loss or independence of the Republic of Texas, the rationale being that Santa Anna was not a representative of Mexico and that he signed away Texas under duress. Mexico declared its intention to recapture what it considered a breakaway province.

In the decade after the war, Texas consolidated its position as an independent republic by establishing diplomatic ties with the United Kingdom and the United States. Most Texans were in favor of annexation by the United States, but anti-slavery Northerners feared that admitting another slave state would tip the balance of national power to the slave-holding South, and they delayed Texas's annexation for almost a decade. Consequently, Texas was not admitted to the union until 1845, when it became the 28th state.

The Mexican government complained that by annexing its "rebel province," the United States had intervened in Mexico's internal affairs and unjustly seized its sovereign territory. The major European powers, led by Britain and France, recognized the independence of Texas and repeatedly tried to dissuade Mexico from declaring war. British efforts to mediate were fruitless because additional political disputes (particularly the Oregon boundary dispute) arose between Britain and the United States.

In 1845, the new U.S. President, James K. Polk, sent diplomat John Slidell to Mexico City in an attempt to purchase Mexico's California and New Mexico territories. U.S. expansionists wanted California to thwart British ambitions in the area and to have a port on the Pacific Ocean, which would allow the United States to participate in the lucrative trade with Asia. Polk authorized Slidell to forgive the $4.5 million owed to U.S. citizens from the Mexican War of Independence and pay another $25 to $30 million in exchange for the two territories.

However, Mexico was neither inclined nor in a position to negotiate, largely because of political turmoil. In 1846 alone, the presidency changed hands four times, the war ministry six times, and the finance ministry sixteen times. According to historian D.F. Stevens, both Mexican public opinion and Mexican political factions and leaders were hawkish on the issue of North American territories. Mexicans opposing open conflict with the United States such as President José Joaquín de Herrera and others were considered traitors. When President de Herrera considered receiving Slidell in order to peacefully negotiate the problem of Texas annexation, he was deposed after being accused of treason and trying to hand over part of national territory.

Military opponents of President José Joaquín de Herrera considered Slidell's presence in Mexico City an insult. After a more nationalistic government under General Mariano Paredes y Arrillaga came to power, the new government publicly reaffirmed Mexico's claim to Texas, and Slidell left in a temper, convinced that Mexico should be "chastised."

Opening hostilities

An agreement between Mexican and U.S. government had established the border between Mexico and Texas at the Nueces River. Texas, though, set the border at the Rio Grande River, giving itself more land. Rival claims over the disputed territory would lead to the Mexican-American War.

President Polk had sent General Zachary Taylor and 1,500 American troops to stay in the border along the Nueces River. Taylor arrived with his forces in July 1845, but then was ordered by President Polk to cross into disputed territory. Taylor marched to Corpus Christi, just north of the Rio Grande, as he did not want to provoke an attack. Then in March 1846, Taylor was ordered to march to the Rio Grande with 4,000 troops. Just one month later, the Mexicans attacked, but Taylor's forces were too much for the Mexicans and he drove them back beyond the Rio Grande River. President Polk took advantage of the skirmish and asked for a declaration of war.

By then, Polk had received word of a skirmish between a small contingent of American troops commanded by Captain Seth Thornton and some two thousand Mexican soldiers under the command of Colonel Anastasio Torrejónwas. The greatly outnumbered U.S. forces surrendered after several hours of fighting. Thornton and several officers were taken prisoner, and this incident along with the rejection of Slidell's diplomatic mission were taken as the casus belli. A message to Congress on May 11, 1846, stated that Mexico had "invaded our territory and shed American blood upon the American soil," and a joint session of Congress approved the declaration of war. Democrats overwhelmingly supported the war, but 67 Whigs voted against it on a key amendment. On the final vote only fourteen Whigs voted no, including first-term Illinois Representative Abraham Lincoln. The United States declared war on Mexico on May 13, 1846, and Mexico declared war on July 7 (sometimes the manifest from President Paredes on May 23 is construed as the declaration of war, but only the Mexican congress had that power).

Whigs in both the North and South generally opposed the war, while Democrats mostly supported it. Whig Abraham Lincoln contested the causes for the war and demanded to know the exact spot on which Thornton had been attacked and U.S. blood had been shed. Whig leader Robert Toombs of Georgia charged the president with "usurping the war-making power [and] seizing a country . . . which had been for centuries, and was then in the possession of the Mexicans. . . . Let us put a check upon this lust of dominion. We had territory enough, Heaven knew." (Beveridge 1:417)

After the declaration of war, U.S. forces invaded Mexican territory on two main fronts. The U.S. war department sent a cavalry force under Stephen W. Kearny to invade western Mexico from Fort Leavenworth, Kansas, reinforced by a Pacific fleet under John D. Sloat. This was done primarily because of concerns that Britain might also attempt to occupy the area. Two more forces, one under John E. Wool and the other under Taylor, were ordered to occupy Mexico as far south as the city of Monterrey.

War in California

After war was declared on May 13, 1846, it took almost two months (mid-July 1846) for definite word of war to get to California. U.S. consul Thomas O. Larkin, stationed in Monterey, on hearing rumors of war tried to keep peace between the Americans and the small Mexican military garrison commanded by José Castro. American army captain John C. Frémont with about 60 well-armed men had entered California in December 1845, and was making a slow march to Oregon when they received word that war between Mexico and the U.S. was imminent.

On June 15, 1846, some 30 settlers, mostly Americans, staged a revolt and seized the small Mexican garrison in Sonoma, raising the "Bear Flag" of the California Republic. It lasted one week until the U.S. Army, led by Frémont, took over on June 23. (The California state flag today is based on this original Bear Flag and retains the words "California Republic.")

Commodore John Drake Sloat, on hearing of imminent war and the revolt in Sonoma, ordered his naval forces to occupy Yerba Buena (present San Francisco) on July 7, and raise the American flag. On July 15, Sloat transferred his command to Commodore Robert F. Stockton, a much more aggressive leader. Stockton put Frémont's forces under his orders, and on July 19, Frémont's "California Battalion" swelled to about 160 additional men from newly arrived settlers near Sacramento. He entered Monterey in a joint operation with some of Stockton's sailors and marines, while American forces easily took over the north of California. Within days they controlled San Francisco, Sonoma, and Sutter's Fort in Sacramento.

In southern California, Mexican General José Castro and Governor Pío Pico fled from Los Angeles. When Stockton's forces entered Los Angeles unresisted on August 13, 1846, the nearly bloodless conquest of California seemed complete. Stockton, however, left too small a force (36 men) in Los Angeles, and the Californios, acting on their own and without help from Mexico, led by José Mariá Flores , forced the small American garrison to retire in late September. More than 200 reinforcements sent by Stockton, led by U.S. Navy Capt William Mervine were repulsed in the Battle of Dominguez Rancho (October 7–9, 1846) near San Pedro, where 14 U.S. Marines were killed. Meanwhile, General Kearny with a much reduced squadron of 100 dragoons finally reached California after a grueling march across New Mexico, Arizona, and the Sonora desert. On December 6, 1846, They fought the Battle of San Pasqual near San Diego, California, where 18 of Kearny's troop were killed—the largest American casualties lost in battle in California.

Stockton rescued Kearny's surrounded forces and with their combined troops, they moved northward from San Diego, entering the Los Angeles area on January 8, 1847. Linking up with Frémont's men and with American forces totaling 660 troops, they fought the Battle of Rio San Gabriel, the next day the Battle of La Mesa. On January 12, 1847, the last significant body of Californios surrendered to American forces. That marked the end of the war in California. On January 13, 1847, the Treaty of Cahuenga was signed.

On January 28, 1847, U.S. Army lieutenant William Tecumseh Sherman and his army unit arrived in California as American forces in the pipeline continued to stream into the territory. On March 15, 1847, Col. Jonathan D. Stevenson’s Seventh Regiment of New York Volunteers of about 900 men start arriving in California. At the conclusion of the war, all of these men would be joined by thousands more when word went out that gold was discovered in January 1848, launching the California Gold Rush.

War in Northeastern Mexico

The defeats at Palo Alto and Resaca de la Palma caused political turmoil in Mexico, which Antonio López de Santa Anna used to revive his political career and return from self-imposed exile in Cuba. He promised the U.S. military leaders that if allowed to pass through their blockade, he would negotiate a peaceful conclusion to the war and sell the New Mexico and California territories to the United States. Once he arrived in Mexico, however, he reneged and offered his military services to the Mexican government. Santa Anna was promptly appointed general, but instead of taking to the field, he seized the presidency.

U.S. troops led by Taylor crossed the Rio Grande after some initial difficulties in obtaining river transport. He occupied the city of Matamoros, then Camargo where, while waiting, the soldiery suffered the first of many problems with disease. He then proceeded south and besieged the city of Monterrey, with Mexican forces under General Pedro de Ampudia. The Battle of Monterrey was a hard fought engagement during which both sides suffered serious losses. The Americans light artillery was ineffective against the stone fortifications of the city. The U.S. infantry division and the Texas Rangers captured four hills to the west of the town and with them heavy cannon. That lent the U.S. soldiers the strength to storm the city from the west and east. Once in the city, U.S. soldiers fought house to house: each was cleared by throwing lighted shells, which worked like grenades. Eventually, these actions drove and trapped Ampudia's men into the city's central plaza, where howitzer shelling forced Ampudia to negotiate. Taylor allowed the Mexican Army to evacuate and agreed to an 8-week armistice in return for the surrender of the city. Under pressure from Washington, Taylor broke the armistice and occupied the city of Saltillo, southwest of Monterrey. Santa Anna blamed the loss of Monterrey and Saltillo on Ampudia and demoted him to command a small artillery battalion.

On February 22, 1847, Santa Anna personally marched north with 20,000 men to fight Taylor and his 4,600 men, entrenched at a mountain pass called Buena Vista. Santa Anna suffered desertions on the way north and arrived with 15,000 men wearied by the forced march. He demanded and was refused surrender of the U.S. army, and then attacked the next morning. Santa Anna flanked the U.S. positions by sending his cavalry and some of his infantry up the steep terrain that made up one side of the pass, while a division of infantry attacked frontally along the road leading to Buena Vista. Furious fighting ensued during which U.S. troops were almost routed, but were saved by artillery fire against a Mexican advance at close range by Captain Braxton Bragg, and a charge by the mounted Mississippi Riflemen under Jefferson Davis. Having suffered discouraging losses, Santa Anna withdrew that night, leaving Taylor in control of Northern Mexico. Polk distrusted Taylor, whom he felt had shown incompetence in the Battle of Monterrey by agreeing to the armistice, and he may have considered him a political rival for the White House. Taylor later used the Battle of Buena Vista as the centerpiece of his successful 1848 presidential campaign.

Scott's campaign

Rather than reinforce Taylor's army for a continued advance, President Polk sent a second army under General Winfield Scott, which was transported to the port of Veracruz by sea to begin an invasion of the Mexican heartland. Winfield Scott became an American national hero after his victories in the Mexican War, and later became military governor of occupied Mexico City. Scott performed the first major amphibious landing in the history of the United States in preparation for the Siege of Veracruz. A group of 12,000 volunteer and regular soldiers successfully offloaded supplies, weapons, and horses near the walled city. Included in the group were later Civil War commanding generals Robert E. Lee and George Meade. The city was defended by Mexican General Juan Morales with 3,400 men. Mortars and naval guns under Commodore Matthew C. Perry were used to reduce the city walls and harass defenders. The city replied as best it could with its own artillery. The effect of the extended barrage destroyed the will of the Mexican side to fight against a numerically superior foe, and they surrendered the city after 12 days under siege. U.S. troops suffered 80 casualties, while the Mexican side had around 180 killed and wounded, about half of whom were civilian. During the siege, the U.S. side began to fall victim to Yellow Fever.

Scott then marched westward toward Mexico City with 8,500 healthy troops, while Santa Anna set up a defensive position in a canyon around the main road at the halfway mark to Mexico City, near the hamlet of Cerro Gordo. Santa Anna had entrenched with 12,000 troops and artillery that were trained on the road, along which he expected Scott to appear. However, Scott had sent 2,600 mounted dragoons ahead, and the Mexican artillery prematurely fired on them and revealed their positions. Instead of taking the main road, Scott's troops trekked through the rough terrain to the north, setting up his artillery on the high ground and quietly flanking the Mexicans. Although by then aware of the positions of U.S. troops, Santa Anna was unprepared for the onslaught that followed. The Mexican army was routed. The U.S. army suffered 400 casualties, while the Mexicans suffered over 1,000 casualties and 3,000 taken prisoner.

In May, Scott pushed on to Puebla, the second largest city in Mexico. Because of the citizens' hostility to Santa Anna, the city capitulated without resistance on May 15. Mexico City was taken in the Battle of Chapultepec and subsequently occupied.

Treaty of Guadalupe Hidalgo

The Treaty of Guadalupe Hidalgo, signed on February 2, 1848 by American diplomat Nicholas Trist, ended the war and gave the U.S undisputed control of Texas, established the United States–Mexican border of the Rio Grande River and ceded more than forty two percent of its pre-war territories to the United States. California, Nevada, Utah, and parts of Colorado, Arizona, New Mexico, and Wyoming were given to the U.S. In return, Mexico received $15,000,000. This exchange is known as the Mexican Cession. Mexicans living in the conquered lands were given the option of returning to Mexico or staying and become American citizens. Part of the Treaty of Guadalupe Hildago, notably Article X, was struck from the treaty before it was ratified by the United States Senate. These articles had promised that the United States would recognize Mexican and Spanish land grants.

Five years later, negotiations began to complete the purchase of what is modern day Arizona and New Mexico. The Gadsden Purchase provided for the payment by the United States of $10,000,000 to the Mexican Government for more than 29,000 square miles (76,900 sq km). By contrast the Mexican Cession yielded approximately 554,000 square miles (1,435,500 sq km).

Although 13,000 U.S. soldiers died during the course of the Mexican War, only about 1,700 were killed in combat. Ninety percent died of disease, such as yellow fever. Mexican casualties are estimated at 25,000.

One of the contributing factors to loss of the war by Mexico was the inferiority of their weapons. The Mexican army was using British muskets from the Napoleonic Wars, while U.S. troops had the latest U.S. manufactured rifles. Furthermore, Mexican troops were trained to fire with their musket held loosely at hip-level, while U.S. soldiers used the much more accurate method of butting the rifle up to the shoulder and taking aim along the barrel.

The Saint Patrick's Battalion (San Patricios), was a group of several hundred, the majority of which were Irish immigrant soldiers who deserted the U.S. Army and joined the Mexican army. Most were killed in the Battle of Churubusco; about 100 were captured and hanged as deserters.

Results

Mexico lost more than 500,000 square miles (1,300,000 square km) of land, almost half of its territory. The annexed territories contained about 1,000 Mexican families in California and 7,000 in New Mexico. A few moved back to Mexico; the great majority remained and became U.S. citizens.

A month before the end of the war, Polk was criticized in a United States House of Representatives amendment to a bill praising Major General Zachary Taylor for "a war unnecessarily and unconstitutionally begun by the President of the United States." This criticism followed congressional scrutiny of the war's beginnings, including factual challenges to claims made by President Polk. The vote followed party lines, with all Whigs supporting the amendment.

In much of the United States, victory and the acquisition of new land brought a surge of patriotism (the country had also acquired the southern half of the Oregon Country in 1846 through a treaty with Great Britain). Victory seemed to fulfill citizens' belief in their country's Manifest Destiny. While Whig Ralph Waldo Emerson rejected war "as a means of achieving America's destiny," he accepted that "most of the great results of history are brought about by discreditable means." Although the Whigs had opposed the war, they made Taylor their presidential candidate in the election of 1848, praising his military performance while muting their criticism of the war itself.

The war had been widely supported by Democrats and opposed by Whigs. Many Northern abolitionists attacked the war as an attempt by slave-owners to expand slavery and assure their continued influence in the federal government. Henry David Thoreau wrote his essay Civil Disobedience and refused to pay taxes to support the war. Former President John Quincy Adams also expressed his belief that the war was an effort to expand slavery. In 1846, Democratic Congressman David Wilmot introduced the Wilmot Proviso to prohibit slavery in any new territory acquired from Mexico. Wilmot's proposal did not pass, but it sparked further hostility between the factions.

Winders, Richard Bruce. Mr. Polk's army the American military experience in the Mexican War. Texas A & M University military history series, 51. College Station, Tex: Texas A & M University Press 1997. ISBN 9780585147406

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